Magnetic recording media are widely used as audio tapes, video tapes, computer tapes, floppy disks, etc. Magnetic recording media have attained higher densities and shorter recording wavelengths, and in addition to the analogue recording mode, the digital mode has also been studied. In order to meet the higher-density requirement, a magnetic recording medium employing a thin metal layer as a magnetic layer has been proposed. This metal thin film type magnetic recording medium is inferior in production efficiency and in reliability for practical use (such as corrosion resistance), to so-called coated-type magnetic recording media produced by coating on a support ferromagnetic particles dispersed in a binder. However, the coated-type magnetic recording medium shows Door electromagnetic characteristics because the coated-type magnetic recording medium has lower packing density of ferromagnetic particles than the metal thin film type magnetic recording medium. The coated type magnetic recording media comprising a nonmagnetic support having thereon a magnetic layer containing particles of ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide, CrO.sub.2, a ferromagnetic alloy, or the like dispersed in a binder is widely used.
The electromagnetic characteristics of coated-type magnetic recording medium can be improved by improving the magnetic properties of the ferromagnetic particles or by smoothing the surface. For attaining these improvements, various methods have been proposed. However none of these proposals is sufficient to attain higher density.
Along with the desire for higher densities, there is a recent trend toward recording wavelength reduction. As a result, the problems of self-demagnetization loss, during recording and thickness loss during reproduction, both of which lead to reduced output, have become severe when the magnetic layer has a large thickness.
Although thickness reduction in magnetic layers has been attempted in order to eliminate the above problems, a magnetic layer thickness of about 2 .mu.m or less is disadvantageous in that the surface of such a thin magnetic layer is apt to be affected by the nonmagnetic support, resulting in impaired electromagnetic characteristics and worsened drop out (DO). This influence of the support surface roughness can be eliminated by first forming a thick, nonmagnetic undercoating layer on the support surface and then forming thereon a magnetic layer as an upper layer, as described in JP-A-57-198536. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) This method, however, has problems in that no improvement is attained in head wear or durability. This may be attributable to the fact that the conventional, nonmagnetic lower layer employs a thermosetting resin as a binder and hence the magnetic layer is brought into contact with a head or with other parts without a buffer due to the cured lower layer, and the magnetic recording medium having such a lower layer has slightly insufficient flexibility.
Although the above problem may be eliminated by using a non-curing resin as the binder for the lower layer, the use of a non-curing resin binder poses problems, for example, the surface roughness of the magnetic layer are impaired which results in poor electromagnetic characteristics. This is because in the conventional method in which a magnetic layer as an upper layer is applied after a lower layer has been applied and dried, the lower layer swells due to the organic solvent contained in the coating solution for forming the upper layer and affects the upper layer-forming coating solution by, for example, causing a turbulent flow therein. A reduction in magnetic layer thickness may be attained by reducing spread rate or by lowering the concentration of a magnetic coating solution by adding a large amount of solvent thereto.
In the case of the former reducing means, due to the reduction in spread rate, the applied coating solution begins to dry before it has sufficiently leveled, thus creating a problem of coating defects remaining in the resulting magnetic layer, such as streaks or a stamp pattern. Consequently, a considerably poor yield results. In the case of the latter reducing means, the low concentration of the magnetic coating solution exerts various negative effects. For example, the coating film formed has a large amount of voids, so that a sufficiently high packing density of magnetic particles cannot be obtained and the coating film has insufficient strength. These serious problems lead to poor yield in the technique disclosed in JP-A-62-154225.
As one method for eliminating these problems, a magnetic recording medium produced by a simultaneous multiple layered coating method as described in JP-A-63-191315 and JP-A-63-187418 has been proposed. By employing the simultaneous multiple layered coating method in which a nonmagnetic layer is formed as a lower layer and an upper magnetic layer containing ferromagnetic particles is formed thereon while the lower layer is in a wet state, a magnetic recording medium can be produced which is free from coating defects, is excellent in production efficiency, and has been improved in electromagnetic characteristics such as reproduction output and C/N and in running durability.
However, even with the above-described coating method, the following problem remains unsolved.
In recent years, magnetic recording media are required to have an extremely smooth surface in order to reduce the spacing loss caused by a gap between the magnetic recording medium and a magnetic head to obtain a higher density and higher output. For attaining this, it is becoming more necessary that the nonmagnetic lower layer, which is not directly exposed, also should have as good dispersion state as possible, and the surface resulting from simultaneous multiple layered coating should be smooth. It may be thought that in the above-described simultaneous multiple layered coating technique, the surface roughness of the magnetic layer can be improved by reducing the size of the particles employed in the lower layer to ensure the smooth surface of the lower nonmagnetic layer. However, use of such fine particles poses a problem in that these particles are apt to aggregate to impair the surface roughness of the lower layer rather than to improve it, and this in turn causes the magnetic layer to have poor surface properties.
The problem is that even when an improvement in electromagnetic characteristics is attempted by further reducing the thickness of a magnetic layer, the interface between the magnetic layer and the lower layer is difficult to control because of the poor dispersibility of the particles in the lower layer and, as a result, the interface becomes rough, and a uniformly smooth magnetic layer cannot be obtained. That is, as a result of a reduction in magnetic layer thickness, the dispersibility in the lower nonmagnetic layer has a greater effect on the surface roughness of the magnetic layer formed by simultaneous multiple layered coating. However, any of the conventional techniques cannot properly eliminate these difficulties.
Thus, although it is necessary to further improve the dispersibility of nonmagnetic particles, a proper means has not been found.
Another problem is that when a magnetic layer has too smooth of a surface, sufficient running durability cannot be obtained because repeated running operations cause sticking or head clogging due to the increased coefficient of friction.
In a well known technique for overcoming this problem in coated-type magnetic recording media, head-cleaning ability is enhanced by increasing the amount of an abrasive agent, or running properties are improved by increasing the amount of carbon black or by using a carbon black having larger particle sizes. Use of these methods, however, results in a lower packing density of ferromagnetic particles and hence adversely influences the attainment of higher density.
As a way to impart good running properties to an especially smooth magnetic layer, a technique of forming protrusions on the magnetic layer surface is described in JP-A-2-81315 and JP-A-2-113423. This method disadvantageous in that it is difficult to diminish noise and to attain both a high level of electromagnetic characteristics and running stability.
Further, a technique is disclosed in JP-A-63-241720 which uses a combination of .alpha.-alumina as an abrasive agent and carbon black in order to improve running properties, surface electric resistance, and mar resistance (i.e., wear resistance). However, the magnetic recording media produced by this technique either have good electromagnetic characteristics with insufficient running properties, or have good running properties with insufficient electromagnetic characteristics. Thus, the above technique has been unable to stably attain both electromagnetic characteristics and running durability without fail. Besides the above reference, the technique of using a combination of an abrasive agent and carbon black is also described in JP-A-2-105324, JP-A-61-194634, JP-A-62-246134, and others. However, both electromagnetic characteristics and running durability have not been stably attained by any of these methods.
It is also known that calendering is conducted under various conditions in order to smooth the surface of a magnetic layer. For example, the following techniques are known: JP-B-52-17404 discloses a magnetic recording medium production process in which surface treatment is conducted at a linear pressure of 50 to 500 kg/cm.sup.2 using a high-hardness elastomeric roll made of, e.g., urethane rubber, and a metal roll heated at 40 to 200.degree. C. (the term "JP-B" as used herein means an "examined Japanese patent publication"); and JP-A-61-24022 discloses a process for producing a magnetic recording medium having a two-layer structure which comprises calendering the first magnetic layer under conditions of a linear pressure of 350 kg/cm.sup.2 or more and a roll temperature of 80.degree. C. or higher and calendering the second magnetic layer under conditions of a linear pressure of 250 kg/cm or more and a roll temperature of 60.degree. C. or higher. Besides these, the techniques disclosed in JP-A-64-76532, JP-A-1 -315025, and others are known. However, the magnetic recording media produced by these techniques either have good electromagnetic characteristics with insufficient running properties, or have good running properties with insufficient electromagnetic characteristics. Thus, the above techniques have been unable to stably attain both electromagnetic characteristics and running durability without fail.